Expression of GST fusion

Hi, I'm using the GST fusion protein expression system and getting fairly
good expression. However, when purifying the protein, a significant amount
of the fusion protein is getting broken down, and I get an enormous band
of GST+extra a.a. running at ~30kD (GST is 26kD) immediately after cell
lysis. I've used a wide array of protease inhibitors which seem to have
little effect. Has anyone had any luck in getting rid of this band?
Thanks,
Gary